The invention is related to a multi-mode electro-mechanical variable speed transmission in a powertrain, and to a method of operating the transmission and the powertrain. More specifically, it is related to a multi-mode electro-mechanical variable speed transmission with load free clutch shifting between different modes of operation. It is applicable to a wide variety of vehicles and power equipment.
To reduce fuel consumption and emission, hybrid vehicles combine an electric power plant with a conventional internal combustion engine. The internal combustion engine operates in a certain range of speed and power. Inside this range, there usually exists a smaller regime where the engine archives the best performance. On the other hand, however, driving conditions vary enormously, not only in wheel speed but also in driving torque at the drive wheels. A combination of a given speed and torque defines a power state. Selectively operating the internal combustion engine and matching its power state with that of the drive wheels are the primary functions for a hybrid transmission.
The development of hybrid technology provides new avenues for achieving improved operation and match of power state of the internal combustion engine with the drive wheels. Among various power-train architectures, a well-known design is the electro-mechanical continuous variable transmission, known as Toyota hybrid system, or THS. THS allows for electric propulsion at low power and slow speed operation and turns on the engine in hybrid operation when vehicle speed and or power demands exceed certain thresholds. In the hybrid operation, THS splits the input power into two different power paths. Part of the input power passes through a so-called mechanical power path which is comprised of mechanical gears and shafts; the rest of the input power passes through an electric power path which contains electric machines, inverters and battery packs linked by electric cables. The device used to split the power is a simple planetary gear system. THS offers one power split mode and provides a single output to input speed ratio node point SR where one the electric machines is zero rotational speed. This node point is referred to as speed ratio node or speed node. When the transmission operates at a speed ratio higher than the speed node, internal power circulation occurs. One of the power paths passes more power than what is transmitted through the transmission. Internal power circulation reduces the efficiency of the transmission and, to a large extent, constrains the effective operating speed ratio range of the transmission. For high power vehicle applications, the torque and power ratings for the electric machines have to be increased significantly. In the electric drive mode, only one of the electric machines provides motive power. This makes THS not suitable for all electric drive in power demanding applications. Examples of such applications are all electric range (AER) plug-in hybrid configurations where the vehicles operate in pure electric mode till the battery charge is depleted below a predetermined threshold.
U.S. Pat. No. 8,734,281, U.S. Pat. No. 9,108,624 and US application 2015/0292600 disclosed multi-mode electro-mechanical variable speed transmissions that overcome the aforementioned drawbacks of prior art. These transmissions provide much improved fuel efficiency and performance characteristics; they can operate under multiple operation modes including two different power split modes, and offer higher power transmission efficiency by avoiding internal power circulation. These transmissions are capable of providing continuously variable output to input speed ratio and independent power regulation with significantly extend the operational speed ratio range. These transmissions may also be operated in pure electric modes with much improved acceleration performance, and fixed speed ratio modes with maximum transmission efficiency.
Switching between different power splitting modes is achieved through a clutch or clutches, referred to as torque transfer device or devices. Hydraulic actuated frictional clutch is often adopted for its smooth engagement and disengagement quality. Frictional clutch allows the components to be connected to engage under sliding conditions. The torque transfer between the connecting components is established gradually during the engagement process. This leads to a smooth shifting between different modes of operation for the transmission. The major disadvantage of frictional clutch is high power loss due to frictional heat generation in the slippage phase of the engagement and parasitic loss due to churning and drag of fluid in the hydraulic system.
General motors' Chevy Voltec is an example of using frictional clutch for mode shifting in the hybrid transmission. The transmission architecture for the second generation Voltec was disclosed in U.S. Pat. No. 8,602,938.
Positive engagement clutch, such as a dog clutch is desirable for its simplicity, high efficiency and high torque capability. However, this type clutch needs speed synchronization between the components to be connected before clutch engagement is commenced. A major disadvantage with positive engagement clutch is the impact load that the transmission experiences when shifting under torque load. For this reason, dog clutches are mostly used in coordination with a frictional clutch, as seen in manual or automated manual transmissions, to provide torque or power interruptive gear shifting.
In U.S. Pat. No. 8,734,281, U.S. Pat. No. 9,108,624 and US application 2015/0292600, shifting between different modes of operation was recommend to take place at speed ratio nodes where components to be connected are self-synchronized. To facilitate the mode shifting and to reduce or avoid impact load related drive torque disturbance, self-synchronization has to be closely maintained. This requires the transmission to keep its output-to-input speed ratio at a constant as close as possible during mode shifting.